Existing string trimmers use a flexible line spinning at high speeds to cut vegetation. The line is often made of a plastic or similar material that is gradually worn down during use. As the line shortens, additional line is released from a spool to maintain a minimal line length for proper operation.
Existing line feed mechanisms for string trimmers fall into two general categories; bump feed and automatic line feed systems. Bump feed systems require the user to bump the working head on the ground, which causes the spool to be release from a locked position so that additional line is fed. The problem with this method is that it requires the user to regularly bang the spool housing on the ground, which wears out the spool housing. Furthermore, accidental line feeding occurs when the user inadvertently bangs the string trimmer on the ground, thus feeding out line when not needed and leading to waste.
Automatic feed systems use a pivoting arm or something similar to balance forces within the spool to feed out line when the forces become unbalanced. The cutting line extends from the spinning spool and generates a force that, over time, decreases as the line is worn down. When the line is shortened past a minimum length, the force generated by the line is below a threshold so that the pivoting arm is released, allowing the spool to feed out additional line. The problem with these automatic feed systems is they are very delicate and establishing the proper balance of forces within the spool is difficult. Oftentimes, cutting line is fed out when not needed. For example, a user will often operate the string trimmer close to a fence or wall, and this wears down cutting line very quickly. Because additional line is being automatically fed, the user is unaware that the line is being worn down so quickly, which this leads to wasted line and empty spools. Conversely, when additional line is needed, oftentimes cutting line is not properly fed out. This leads to a very frustrating user experience.